Oxidative and Internal Stress Effects of Dopants in Multilayer Yb₂Si₂O₇ Environmental Barrier Coatings

Citation

Herren, Benjamin Riley (2022) Oxidative and Internal Stress Effects of Dopants in Multilayer Yb₂Si₂O₇ Environmental Barrier Coatings. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4cm0-8e79. https://resolver.caltech.edu/CaltechTHESIS:06012022-214247822

Abstract

One of the best approaches to improving the efficiency of turbine engines is to increase their operating temperatures. A revolutionary improvement toward this goal will be the replacement of structural metallic components with silicon-based ceramic-matrix composites (CMCs). However, corrosive chemistries in combustion engines necessitate chemical protection of the structural material. Environmental Barrier Coatings (EBCs) are designed for this purpose through thermomechanical and chemical compatibility with the system. High-temperature, humid oxidation of the EBC bondcoat, a silicon layer which adheres the topcoat to the substrate, remains a critical failure mode EBC systems.

This work studies the effects of chemical modifications on bondcoat oxidation during steam cycling in a current state-of-the-art EBC (Yb₂Si₂O₇/Si), and the implications for the durability of the system. Chemically modified EBCs have been shown to decrease oxide growth by more than 85 percent after 1000 hours of steam cycling. Post-exposure analyses are used to investigate the effects of chemistry on thermal oxide thickness and microstructure. Synchrotron X-ray scattering at the Advanced Photon Source, Argonne National Laboratory, is used to observe internal strains, connected to oxidation and chemistry through microstructure. Compared internal stresses, oxidation, and microstructure between baseline and modified EBCs promote the effectiveness of topcoat chemical modifications as they may apply to EBC durability. Microindentation, nanoindentation, and ongoing beam-bending experiments are also used to assess EBC interface toughness in a baseline EBC system. Additionally, a custom induction furnace has been designed and tested for in-situ steam cycling at the synchrotron. Traits facilitating the use of the custom furnace at the synchrotron make for convenient steam-cycling and other exposures in conventional laboratory settings, as well, with enhanced customizability and flexibility.

Thesis Files